Voice silencer



Sept. 4, 1951 1.. BERANEK 2,566,975

VOICE SILENCER Filed Jan. 18, 1947 2 Sheets-Shem, 1

1310mm.- ba' M M Sept. 4, 1951 L. L. BERANEK 2,566,975

VOICE SILENCER Filed Jan. 18, 1947 2 Sheets-Sheet 2 o' d m 20 En: Ii u 2 IO 3 Volume of SpeecI IncIosure Q 50 0.0. g Lu 2 a o 0 0 0.3 k o --IO 2 E a o 8 m o O 560 TOO 840 I000 50 I3|O I480 I660 I830 202G FREQUENCY BANDS OF EQUAL CONTRIBUTION TO SPEECH INTELLIGIBILITY d @060 5; 2o 1 m A OLDER DESIGN H B NEW INVENTION lll- I0 I B 11 AREA OF N IMPROVEMENT m Kg 0 3 g u A w 3 lo 9 :o om (DD O 560 700 840 I000 H5O I3IO I480 I660 I830 2020 FREQUENCY BANDS. OF EQUAL CONTRIBUTION TO SPEECH INTELLIGIBILITY Patented Sept. 4, 1951 UNITED STATES PATENT OFFICE VOICE SILENCER. Leo L. Be'ranek, Cambridge, Mass. Application January 18, 1947, SerialNo. 722,831'

This invention is primarily concerned with an acoustic filtering and shielding device for use in speech transmission. More particularly, the invention relates to an arrangement of sound insulating bodies and an acoustic filter combined with transmitter devices such as telephones so as to produce a. selective silencing system by means of which words spoken into the transmitter arecommunicated in a highly intelligible state to a party listening at the receiver station; while to those listening at all other points, and especially in the immediate vicinity of the transmitter, the spoken words are rendered indistinct and effectively muffled.

Speech asproduced by the human voice is made up of relatively low frequencycomponents, i. e. frequencies below 500 cycles per'second, and relatively high frequency components; i. e. frequencies above 500 cycles per second, running up to 15,000 cycles per second or higher. It has been determined that those frequencies lying between 500 and 2000 cycles per second contribute most' to the intelligibility of speech.

These facts are of significance in connection with silencing devices heretofore proposed in the art; Thus it has been proposed to reduce the intensity of speech, asheard from points outsidev of the immediate vicinity of the transmitter, by providing various types of mouth pieces, all of which are designed to surround the transmitter 6 Claims; (Cl. 181-34) L face immediately surrounding the mouth and thus secure a sealing effect.

In the use of such devices several difiiculties' are present; It is known that a mouthpiece may seriously reduce intelligibility owing to the fact that the low frequencies of the voice are appreciably intensified by the presence of the enclosed volume of air in the mouthpiece casing.

This intensification, if allowed to become too an inclosure volume to intelligibility levels of transmitted speech and consisting essentially'in the effect of such inclosure volume upon some of the high frequencies of the voice.

An-object of the invention therefore is to deal with the problems indicated and to provide an improved voice silencing structure which includes means for-carrying on-muted speech transmission without departing from satisfactory intelligibility levels and without subjecting; the user of such instruments to unpleasant back-pressure effects. It is also an'object of theinvention tocombine an acoustical element with a voice-silencing casing in a manner such that undesirable sounds may be excluded where speech is to be trans- .r -mitted from regions of high' ambient noise.

Other objects will appear from the following description. 7

In seeking means-to realize these objectives to the fullest extent, I'have discovered that. in the. important voicefrequencyrange notedabove as lyingbetween 500 and ,2000 cycles. per second," an increase in volume of inclosureproduces an unexpectedly pronounced dropin intelligibility levels. I have also: determined that there isa reasonably well. defined; optimum inclosure volI- ume, consistent with adequate volume for speech production, where interference with. frequencies.

in the 500to 2000 cycle per second. range isat a minimum. 7

With the foregoing, considerations in mind, I have devisedavoice silencing unit which ineludes a unique acoustic filter element combined with a hollow casing andsound absorbent bodies in such amanner astoprovide, onthe one hand, low frequency. filtering and sound. absorption with suitable air outlets; and on. the other hand, a speech inclosure characterized by a relatively small volume, so. chosen asto beacoustically suitedv tov the more important voicefrequencies" lying in a frequency range of from 500 to 2000 cycles per second. i

The voice silencing unit is constructed with, a transmitter receptacle formed inthe casing; andis adapted to be detachablyfastened-to the'trans mitter end of a telephone'of thetype'commonly referred to as a hand set. The casing memberincludes a rigid thin-walled; body of relatively high acoustic: insulating value, having a speech inclosure which.-v communicates with the transmitter receptacle;v The upper part. of the inclosure is defined by concave edges of the casing adaptedto conform t'o the shape of' the face at those portions: immediately surrounding thev mouth; The. lower portionrof' the inclosure is The inclination of the plate is designed so that these openings are uppermost and nearest to the Y mouth and so that speech energy entering the speech inclosure may to a great extent be directed first against these openings. This arrangement furnishes an opportunity for the plate to exercise an excellent filtering action and to separate low frequencies of speech from the high frequencies with the former passing through the passageways in accordance with acoustic filterprinciples.

The close proximity of the passages to the mouth when thedevice isin use, taken in conjunction with the fact that the passageways are designed so as to be short in length, results in rapid passing of the breath through the plate with a minimum of inertial effects and thereby substantially eliminates back pressure and facilitates breathing. With the filter plate in the inclined positiondescribed, the speech energy in the high frequency region entering the inclosure from the mouth is directed against the plate at an angle less than 90 degrees and is then reflected in the direction of the microphone in a highly efllcient manner.

The unusual nature of the filter construction lends itself to formation of an exceptionally effective sound absorbing chamber in which an air duct is brought up to a point where it is closely adjacent to the mouth and may rapidly carry away expelled air. At the same time the duct is made to follow a curved path along either side of which are arranged masses of acoustic material against which the, sound must be deflected. Because of the curved nature of the duct, sound waves, which tend to travel in'straight lines, are certain to penetrate into the masses of acoustical material. Moreover the acoustical properties of the absorbing material are chosen in such a way that they are matched to the volume of the sound absorbent chamber and afford a maximum deadening effect. v

The accompanying drawings illustrate a preferred embodiment of these features, of which Fig. 1 is a plan view of the voice silencer of the invention, looking directly down into the speech inclosure;

Fig. 2 is an end elevational view looking through the receiver opening into the speech inclosure;

Fig. 3 is a symmetrical vertical cross-section taken centrally of the voice silencer, with the transmitter end of a telephone shown engaged in an operating position; and

Figs. 4 and 5 are graphs representing acoustic values.

The general arrangement of speech inclosure receptacle and sound absorbing chamber is apparent from an inspection of Fig. 3. Onev important feature of the structure shown is the choice of volume of the speech inclosure. I find that a volume ,of from 100 to 300 cubic centimeters, such as hascommonly been employed on mouthpieces, results in a loss of from to 20.

.rubber, Bakelite or similar substances.

decibels in the important frequency range between 500 C. P. S. and 2000 C. P. S. The need for a small volume of speech inclosure of a substantially optimum value, is evidenced by the curves illustrated in Fig. 4. The data employed in the curves were obtained from a standard type of acoustical testing procedure on which speech energy from an artificial voice was supplied to a telephone transmitter, using three different volumes of speech inclosure. Sound pressure established at the transmitter was determined as a function of frequency.

. These figures indicate the desirability of decreasing the speech inclosure volume below 100 cubic centimeters, and I have found that the volume may successfully be decreased down to approximately cubic centimeters. From a practical standpoint it is not desirable to reduce the volume much below 50 cubic centimeters since smaller volumes will not provide adequate space for the lips and the transmitter of the telephone. A volume of substantially '75 cubic centimeters was employed in the speech inclosure described below.

Referring more in detail to the drawings, Ill denotes a hollow casing member which is formed of an acoustic insulating material such as hard The casing is formed with an irregularly curved undersurface from which extend two opposite flat sides l2 and M. Upper casing portions I6 and I8 are curved inwardly to form a mouthpiece and terminate along concaved edges 20 which define an aperture opening into a speech inclosure 22 as may be more clearly seen in Fig. 3.

At points immediately adjacent to the speech inclosure the casing body. is formed with a semicircular frame portion 24. Arranged belowthe frame portion 24 is a second semi-circular frame portion 26 which forms a part of a separate casing section 28 secured to the main body of the casing by means of bolts 30 engaged in threaded projections 32. Located around the inner peripheral surfaces of the frame portions 24 and 26 are strips 34 and 3B of sound absorbent material such as felt. Also attached to the frame portions at separated intervals are spring clips 38. The frame portions, strips and spring clips combine to form a transmitter receptacle in which the transmitter end 40 of a hand set may be detachably secured in an inclined position such as has been illustrated in Fig. 3.

The separate casing section 28 is of a generally rounded shape as is indicated in Figs. 1 and 2 and extends outwardly below the transmitter end of the receiver to substantially increase the amount of space within the casing. Through the end of the casing section 28 are formed outlets 42 and against these openings at the inner side of the casing material is arranged a screen member 43 of wire mesh or other reticulated body, overlying the area included by the outlets 42.

Numeral 44 denotes the acoustic filter element of the invention, consisting of a thin plate preferably composed of a material such as fibreboard maintaining the plate in an inclined position may however bev desired to be employed.

assume At the uppermost'edge of the plate 44, as viewed" in Fig. 3, I have further provided openings 48 arranged in a curved path as shown, in a position such ,that they occur very near to the mouth. These openings are of such a diameter that the plate can be formed of a thin material and yet act as a suitable filter element so that the length of openings 48 is very short, a feature which is essential in reducing the back pressure of the acoustic filter element.

The plate 44 serves not only as a filter element and'reflecting surface but also acts as a dividing wall for separating the casing cavity into the speech inclosure 22 and a lower sound absorbing chamber. This sound absorbing. chamber extends upwardly to the under side of the plate 44 and therefor air and low frequencies are furnished with a highly accessible path through which they may rapidly escape from the speech inclsoure.

Within the sound absorbing chamber are combinedsections or layers of sound insulating material 50' and 52 which occupy a substantial part of the space therein. The two layers 50 and 52 are arranged in spaced-apart relation and supported by two reticulated elements 54 and 56 which may for example consist of wire screening bent into a curved formation as noted in Fig. 3, to define a curved air duct 58. The upper extremities of the screen elements extend to a point just: below the plate 44" and a space 68 is maintained just below the openings 48 so that the latter openings directly communicate with the air duct 58, whichin turn leads to th screen 43 and outlets 42, forming a continuous passageway.

In accordance with the invention, the filter orifices are selected so that their positive acoustical reactance is justequal to and, hence, cancels the negative acoustical reactance of the inclosed volume of the sound absorbing chamber. The method by which I determine. the proper size and number of holes in the'filter plate, necessary to bring the net reactance to-zero, is as follows:

Use is made of a publication standard in the acoustic art entitled: Acoustic Design Charts by Frank Massa, published by the Blakiston Co., 1942. In particular, reference is made to charts numbered 41 and 43 therein. Chart No. 41 sets forth the acoustic reactance (positive) of a circular orifice formed in a wall or, partition whose thickness is small compared with the diameter of.

the orifice. If more than oneorifice is used, the reactance is divided by the number of. holes. Chart No. 43 gives the acoustic reactance (nega-. tive) of an inclosed volume whose. maximum linear dimension is small compared with the wave length of sound at the frequency or" operation.

In designing a filter, the frequency at which the filtering starts to take place, i. e. the change-over frequency, is found when the acoustic re-actance (positive) of the holes equals the acoustic reactance (negative) of the inclosed volume of the sound-absorbent chamber. In the case of the telephone silencer of the invention, the changeoverfrequency was chosen at approximately 500 C. P. S. Calculations made from the above noted charts furnish the data that four orifices, each having a diameter of es of an inch, would bring the net reactance t'o'zero and; hence, would make a suitable filter when used with an inclosed volume of 305) cubic centimeters;

Having selected thediameter of the orifices, it is necessary to provide that the thickness of the platein which they are formed be smallcompared with the diameter of'the orifices. If the thickvaried to some extent.

In figuring the volume of the inclosed air space in'the sound-absorbing chamber, the effect of the added absorbing material should be included because chart No. 43 assumes that no absorbing;

material is present. I have'foundqthatthe actual volume of the inclosed space must be considered" to be'enlargedby approximately 40per cent when it is filled with acousticalmaterial of the type specified in the preferred embodiment of my invention, if chart No. 43 is to be applicable. Some variance in this percentagefigureispermissible,

Material supporting this fact is found in an" article under my name entitled: Acoustic Impedance of Porous Materials, published in. the Journal of the Acoustical Society of America, vol. 13, pages 248-260, January 1942. actual volume of the inclosed air space should;

for example, be approximately 250 cubic centi,

meters where four orifices, having a diameter of. of an inch, are employed. Similar reducing of the net reactance to zerocan'be achieved, using other volumes which would require orifices having. different diameters and employed in different numbers.

The acoustical absorbing material chamber.

absorbing material placed therein. An acoustical.

material can be specified in terms of. weight,- thickness and more particularly by its fiow-re.-.. The latter term denotes a character.- istic of the. material which isrelatedtothe.degree" ofoppositionthat the materialofiers to. the flow.

sistance.

of air throughit.

In determining an optimum. range of. values for the flow resistance of the material. to be. placed in my sound-absorbing chamber, the vol.- ume of the chamber was takento. be 250. cubic centimenters with a d'epthof approximately three.v

Larger volume could. be, used butpractical limitations on size and weight forced a,

inches.

compromise.

The acoustical impedance of an inclosurein.-

which there. is. an acoustical material is known to be.v as follows: i

Acoustical impedance Rid/3 centimeter thickness of the material; d is the depth of the chamber in centimeters; is the frequencyin cycles per second at which absorption is desired; a: (-1) and Z is the acoustical impedance.

The-absorption of the sound in the inclosure is given bythe well known formula:

Rf (3/d) [(1764) (230,000/fd) In order that aninclosure 3 inches (7.62 cm.).- in depth absorb soundin an optical manner in Hence the constituting the layers 50 and 52 are chosen so that they are; matched to the volume of the sound-absorbing By this ismeant that for. any given. size of chamber there isan optimum range of' values for the flow resistance of the acousticv yrzsopoo/rdy where Rf is the acoustical flow. resistance of one.

weare- 7.. the frequency range between 50 and 150 cycles per second, which is where the voice frequencies that are most diflicult to absorb are located, the flow resistance Rf should be between 80 and 200 acoustic ohms per centimeter of thickness and preferably one-half way between. In the device constructed and described above, Fiberglas AA, manufactured by the Owens-Corning Fiberglas Corporation, Toledo, Ohio, was used. This is a material made of glass fibers whose mean fiber diameter is about one micron and Whose flow resistance lies in the range indicated. Other substances whose fiow resistance is found to be in this range might also be employed.

In operation the device is snapped on over the transmitter end of a hand set in the manner indicated in Fig. 3, and the concaved edges 20 are placed firmly against the face. A telephone conversation is then carried on in the usual manher, with words being spoken directly into the speech inclosure 22. As this occurs, the breath and air driven by speech energy are instantly received through the openings 48 which are lying just below the lips and are rapidly passed along the duct 58 and relased through the air outlets 42. Two factors here contribute to substantial elimination of back pressure such as is frequently experienced in mouthpiece devices. First, the relatively short lengths of the holes 48, governed as they are by the thickness of the plate 44, allow air to be transferred from the speech inclosure to the sound-absorbing chamber without the usual inertial effects which have commonly been encountered where relatively longer passageways were employed. Secondly the location of the openings almost directly before the lips of the speaker and very close to them, initiates travel of the air at an earlier point than has heretofore been attempted, with consequent avoidance of time lag and opportunity for back pressure to build up.

Of the frequencies which enter the speech inclosure, the low frequencies, 1. e. those under 500 cycles per second, are passed through the openings 48 which in this case, in combination with the sound-absorbing chamber, function in the well known manner of an acoustic filter. Here also the close proximity of these filter openings to the mouth enables the low frequencies to be very rapidly separated and passed into the soundabsorbing chamber which minimizes the possibility of such low frequencies becoming intensified by the casing to cause distortion. As they enter the chamber, the low frequencies pass to the opening 60, then through the screen elements 54 into the air duct 58. Here the sound waves are causedto strike against and to penetrate the sound-absorbent layers 50 and 52 with the curved formation of the duct insuring that passage of the sound rays will constantly be interrupted to provide an exceedingly efiicient absorbing or deadening action.

Those frequencies of speech entering the speech inclosure which are not thus filtered out are caused to strike against the inclined plate 44 at an angle less than 90 and are almost instantly reflected in thedirection of the transmitter, to yield as satisfactory speech intensity as would normally be the case by speaking directly into the transmitter. V

The total improvement in quality of transmit'- ted speech effected through the principles embodied in the claims of this specification is shown in the graph of Fig. 5. The upper curve B of that graph showns the intensity (sound pressure level) of speech transmitted to thediaphragm of the telephone transmitter when the speech silencer described herein was tested. The lower curve A was obtained in the same manner except the most successful of previous telephone si-, lencers was tested. It is seen that on the average the intensity of the transmitted speech for the invention described herein is 10 to 15 decibels higher than that for the older transmitted speech lever I of this magnitude, in the important frequency range between 500 and 2000 cycles per second, will increase the percentage of Words heard correctly by the listener from 40% (using the old device) to (using the invention described herein).

I claim:

1. An article of the character described, comprising a hollow casing Which is formed with a receptacle for a telephone transmitter, said c'as ing also presenting a speech inclosure communieating with the receptacle, an acoustic element supported in the inclosure and consisting of a substantially rigid thin partition, said partition lying in an inclined position in the path of speech energy delivered to the inclosure, a portion of the partition being formed with low frequency sound sound-absorbing chamber, reticulated membersarranged in contact with the sound-absorbing elements adapted to support them in a curved formation.

2. An article of the character described comprising a hollow casing which is formed with a receptacle for a telephone transmitter, said casing also presenting a speech inclosure communicating with the receptacle, an acoustic element supported in the inclosure and consisting of a thin plate, said plate lying in an inclined position in the path of speech energy delivered to the inclosure, a portion of the plate being formed with low frequency sound filtering openings of relatively short length, said casing having a sound-absorbing chamber communicating with the openings in the said plate, a plurality of sound-absorbing elements disposed in the soundabsorbing chamber, said sound-absorbing elements lying in spaced-apart relation to define a discreet sound absorbing duct, one side of the hollow casing closely adjacent to an outlet end.

with a speaking aperture, an acoustic element forming one side of said inclosure and comprising a substantially rigid thin partition positioned so as to tend to reflect toward said receptacle high frequency components of the sound delivered through said aperture, said partition having a plurality of low frequency sound filtering openings extending therethrough, the casing includinstrument. Under the conditions of listening at the far end' of a long distance telephone line, an increase in ing a sound absorbing chamber communicating through the partition openings with the speech inclosure, the sound absorbing chamber having breath outlets leading therefrom, and low frequency sound absorbing material disposed in the sound absorbing chamber in spaced-apart formation to form a discreet duct through the sides of which the filtered low frequency sound may pass and be absorbed and through which the breath of the talker may be passed from said partition openings to said breath outlets.

4. A voice silencer comprising a hollow casing formed with a receptacle for a telephone transmitter, the casing having provided therein a speech enclosure communicating with the receptacle and formed with a speaking aperture, said speech enclosure having a volume between 50 and 150 cubic centimeters, an acoustic element forming one side of said enclosure and comprising a substantially thin partition positioned at an angle so as to tend to reflect toward said receptacle high frequency components of the sound delivered through said aperture, said partition having a plurality of low frequency sound filtering openings extending therethrough and communicating with a sound absorbing chamber, the dimensions of the filtering openings being so chosen that the positive acoustical reactance of all of the orifices substantially equals and tends to cancel the negative acoustical reactance of the sound-absorbing chamber volume at some frequency lying between 200 and 1000 cycles per second, the sound absorbing chamber having breath outlets leading therefrom and low frequency sound absorbing material disposed ill the; sound absorbing chamber in spaced-apart formation to form a discreet duct, through the sides of which the filtered low frequency sound may pass and be absorbed and through which breath of the talker may be passed from said partition openings to the breath outlets.

5. An article of the character described comprising a hollow casing which is formed with a receptacle for a telephone transmitter, the casing having formed therein a speech enclosure communicating with the receptacle and provided with a speaking aperture, the speech enclosure having a volume of from 50 to 150 cubic centimeters, an acoustical element forming one side of said enclosure and simultaneously one side of a sound absorbing enclosure, said acoustical element comprising a substantially thin partition positioned at an angle so as to reflect toward said receptacle or the frequency components delivered through said speaking aperture, and having a plurality of low frequency sound filtering openings extending therethrough, the dimensions of said filtering openings being so chosen that the positive acoustical reactance of all of the orifices substantially equals and tends to cancel the negative acoustical reactance of the sound absorbing chamber volume at some frequency lying between 200 and 1000 cycles per second, said frequency being the change-over frequency of the acoustical filtering action, the sound absorbing chamber having breath outlets leading therefrom and containing sound-absorbing material, the sound absorbing material having a flow resistance optimal for the absorption of sound in the frequency range between 50 and 300 cycles per second and being disposed in the sound absorbing chamber in spaced-apart formation to form a discreet duct through the sides of which filtered low frequency sound may pass and be absorbed, and through which the breath of the talker may be passed from said partition openings to said breath outlets.

6. A voice silencer comprising a hollow casing which includes a receptacle for a telephone transmitter, a sound absorbing chamber and a speech enclosure, means located between the speech enclosure and the sound absorbing chamber for selectively passing low frequency sound from the speech enclosure to the sound absorbing chamber and reflecting high frequency sound toward said receptacle, and said chamber having a curved duct provided with acoustically porous sides along which the low frequency sound may be absorbed and the breath of the talker passed out of the casing.

LEO L. BERANEK.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 359,492 Balloch Mar. 15, 1887 2,028,180 Arnold Jan. 21, 1936 2,131,820 Scher Oct. 4, 1938 2,225,509 Schober Dec. 17, 1940 2,245,724 Scher June 17, 1940 2,456,346 Veneklasen Dec. 14, 1948 FOREIGN PATENTS Number Country Date 537,196 Germany Oct. 30, 1931 

